Thus RBCs are terminally differentiated; that is, they can never divide. They live about 120 days and then are ingested by phagocytic cells in the liver and spleen. Most of the iron in their hemoglobin is reclaimed for reuse. The remainder of the heme portion of the molecule is degraded into bile pigments and excreted by the liver. Some 3 million RBCs die and are scavenged by the liver each second.

Red blood cells are responsible for the transport of oxygen and carbon dioxide.

Under the conditions of lower temperature, higher pH, and increased oxygen pressure in the capillaries of the lungs, the reaction proceeds to the right. The purple-red deoxygenated hemoglobin of the venous blood becomes the bright-red oxyhemoglobin of the arterial blood.

Under the conditions of higher temperature, lower pH, and lower oxygen pressure in the tissues, the reverse reaction is promoted and oxyhemoglobin gives up its oxygen.

The pressure of oxygen in the lungs is 90–95 torr; in the interior tissues it is about 40 torr. Therefore, only a portion of the oxygen carried by the red blood cells is normally unloaded in the tissues. However, vigorous activity can lower the oxygen pressure in skeletal muscles below 40 torr, which causes a large increase in the amount of oxygen released. This effect is enhanced by the high concentration of carbon dioxide in the muscles and the resulting lower pH (7.2). The lower carbon dioxide concentration (and hence higher pH) at the lungs promotes the binding of oxygen to hemoglobin and hence the uptake of oxygen.

Temperature changes also influence the binding of oxygen to hemoglobin. In the relative warmth of the interior organs, the curve is shifted to the right (like the curve for pH 7.2), helping to unload oxygen. In the relative coolness of the lungs, the curve is shifted to the left, aiding the uptake of oxygen.

Although the oxygen transported by RBCs make possible cellular respiration throughout the body, RBCs lack mitochondria and so cannot perform cellular respiration themselves and must subsist on glycolysis.

Carbon dioxide (CO2) combines with water forming carbonic acid, which dissociates into a hydrogen ion (H+) and a bicarbonate ion:

CO2 + H2O ↔ H2CO3 ↔ H+ + HCO3−

95% of the CO2 generated in the tissues is carried in the red blood cells:

It enters (and leaves) the cell by diffusion through the plasma membrane.

Once inside, about one-half of the CO2 is directly bound to hemoglobin (at a site different from the one that binds oxygen).

The rest is converted — following the equation above — by the enzyme carbonic anhydrase into

bicarbonate ions that diffuse back out into the plasma and

hydrogen ions (H+) that bind to the protein portion of the hemoglobin (thus having no effect on pH).

The bicarbonate ions pass out of the red cell by facilitated diffusion through transmembrane channels in the plasma membrane.

Only about 5% of the CO2 generated in the tissues dissolves directly in the plasma. (A good thing, too: if all the CO2 we make were carried this way, the pH of the blood would drop from its normal 7.4 to an instantly-fatal 4.5!)

When the red cells reach the lungs, these reactions are reversed and CO2 is released to the air of the alveoli.

Although bone marrow is the ultimate source of lymphocytes, the lymphocytes that will become T cells migrate from the bone marrow to the thymus [View] where they mature. Both B cells and T cells also take up residence in lymph nodes, the spleen and other tissues where they

The most abundant of the WBCs. This photomicrograph shows a single neutrophil surrounded by red blood cells.

Neutrophils squeeze through the capillary walls and into infected tissue where they kill the invaders (e.g., bacteria) and then engulf the remnants by phagocytosis.

This is a never-ending task, even in healthy people: Our throat, nasal passages, and colon harbor vast numbers of bacteria. Most of these are commensals, and do us no harm. But that is because neutrophils keep them in check.

However,

heavy doses of radiation

chemotherapy

and many other forms of stress

can reduce the numbers of neutrophils so that formerly harmless bacteria begin to proliferate. The resulting opportunistic infection can be life-threatening.

The number of eosinophils in the blood is normally quite low (0–450/µl). However, their numbers increase sharply in certain diseases, especially infections by parasitic worms. Eosinophils are cytotoxic, releasing the contents of their granules on the invader.

Ordinarily representing less than 1% of the WBCs, their numbers also increase during infection. Basophils leave the blood and accumulate at the site of infection or other inflammation. There they discharge the contents of their granules, releasing a variety of mediators such as:

Platelets are cell fragments produced from megakaryocytes. These polyploid (128n) cells in the bone marrow send pseudopodia-like projections into the lumen of adjacent blood vessels. Blood flowing through the vessel shears off the platelets.

Blood normally contains 150,000–400,000 per microliter (µl) or cubic millimeter (mm3). This number is normally maintained by a homeostatic (negative-feedback) mechanism [Link].

When blood vessels are cut or damaged, the loss of blood from the system must be stopped before shock and possible death occur. This is accomplished by solidification of the blood, a process called coagulation or clotting.

Therefore gamma globulins become more abundant following infections or immunizations.

If a precursor of an antibody-secreting cell becomes cancerous, it divides uncontrollably to generate a clone of plasma cells secreting a single kind of antibody molecule. The image (courtesy of Beckman Instruments, Inc.) shows — from left to right — the electrophoretic separation of:

normal human serum with its diffuse band of gamma globulins;

serum from a patient with multiple myeloma producing an IgGmyeloma protein;

serum from a patient with Waldenström's macroglobulinemia where the cancerous clone secretes an IgM antibody;

Gamma globulins can be harvested from donated blood (usually pooled from several thousand donors) and injected into persons exposed to certain diseases such as chicken pox and hepatitis. Because such preparations of immune globulin contain antibodies against most common infectious diseases, the patient gains temporary protection against the disease. [More]

Most of these tests are performed with enzyme immunoassays (EIA) — Link — and detect antibodies against the agents. However, it takes a period of time for the immune system to produce antibodies following infection, and during this period ("window"), infectious virus is present in the blood. For this reason, blood is now also checked for the presence of the RNA of these RNA viruses:

HIV-1

hepatitis C

West Nile virus

by the so-called nucleic acid-amplification test (NAT).

Thanks to all these precautions, the risk of acquiring an infection from any of these agents is vanishingly small. Despite this, some people — in anticipation of need — donate their own blood ("autologous blood donation") prior to surgery.

Blood Typing

Donated blood must also be tested for certain cell-surface antigens that might cause a dangerous transfusion reaction in an improperly-matched recipient. This is discussed in a separate page — link to it.

Years of research have gone into trying to avoid the problems of blood perishability and safety by developing blood substitutes. Most of these have focused on materials that will transport adequate amounts of oxygen to the tissues.

Some are totally synthetic substances.

Others are derivatives of hemoglobin.

Although some have reached clinical testing, none has as yet proved acceptable for routine use.